It has long been desirable throughout the history of geophysical exploration to measure subsurface parameters such as pressure or temperature gradients as a function of depth with an extremely high degree of resolution, at times even approaching the order of 1 part per million. Accordingly, many widely varying solutions to the difficult problems associated with achieving high resolution subsurface measurements have been attempted with varying degrees of success.
One such solution has been to provide an extremely accurate parameter transducer downhole and to send the raw transducer output, after amplification, uphole via a conventional logging cable. However, this attempted solution has been fraught with numerous difficulties, not the least of which is the signal-to-noise-problems associated with transmission of the instrumentation signal along the logging cable, which is for example, susceptible to induced noise. This, in turn, is due to the long borehole increments associated with modern deep wells over which the cable is suspended--sometimes extending several miles into the earth formations. Yet another problem with such an approach was that the instrumentation signal would thus occupy the entire band width of the cable while the signal was being transmitted uphole in order to avoid cross-talk problems with other signals and the like.
In an effort to circumvent the aforementioned problems encountered in attempting to send the raw transducer signal uphole, it was suggested, particularly with the advent of modern analog-to-digital converters capable of withstanding the extremely deleterious environments associated with subsurface formations, that the transducer signals be converted downhole from analogto-digital form. In theory, this solution appeared attractive in that once the parameter was digitized, modern digital telemetry systems could convey the information uphole without regard to deterioration due to signal to noise problems. Yet another apparent benefit to this approach seemed to be that by monitoring the signals downhole, and digitizing the results for transmission uphole, significant reductions could be achieved in cable band width dedication which has become extremely valuable with the growth in complexity and number of downhole measurments being made with modern logging systems. However, in practice, this approach also proved to be unsatisfactory simply because the state of the art in analog to digital converters suitable for downhole applications has not provided such a converter capable of resolutions on the order of one part per million.
Yet another approach to the problem of obtaining highly accurate subsurface measurement resolution has been attempted as represented typically in a commercially available Model 2811B Quartz Pressure Gauge System manufactured by Hewlett-Packard. In this approach a quartz pressure-sensitive probe is made to oscillate at a nominal resonant frequency in the megahertz range and is mixed with a quartz reference crystal oscillator to generate a different frequency between the 7 to 25 kilohertz frequency range (well within a modern well logging cable band width), whereby the precise difference frequency is a function of the pressure proximate the quartz pressure probe. This different frequency is placed upon the logging cable and by means of a phase locked loop on the surface is multiplied by a factor on the order of 70 for purposes of achieving a pressure sensitity of approximately 105 hertz per PSI. The output of the phase locked loop can be conventionally run to a general purpose frequency counter which may thus measure pressure changes as small as 0.01 PSI in a one second sample of the counter.
While relatively high resolutions may thus be made with the aforementioned approach, in applications wherein such a pressure measuring tool is being run in conjunction with other logging tools in need of cable time, and particularly when digital telemetry is available for encoding the other logging tool measurements for transmission uphole, it has been found most undesirable to have to dedicate a portion of the cable time to transmission of the frequency uphole which is varying as a function of pressure changes. Yet another drawback with such a hybrid approach, wherein digital telemetry data corresponding to data derived from some logging tools is transmitted uphole during one time interval and a frequency output from the pressure tool is transmitted during yet another time interval is that separate hardware is required at the surface for receiving the digital telemetry data on the one hand and the pressure signal on the other.
Thus, it may be appreciated that it would be highly desirable to provide a method and apparatus for measuring downhole various loging parameters with a high degree of resolution wherein the measurement could be encoded in a digital result amenable to transmission in a frame of digital data with other logging parameters. This would obviate the aforementioned problems of cable utilization caused by previous approaches wherein the high resolution data is sent directly uphole. At the same time it would further be highly desirable in such as system to obviate the hereinbefore noted need for an analog-to-digital converter downhole having the desired high resolution, which is practically unfeasible.
The disadvantages of the prior art including those hereinabove recited are overcome by the high resolution measurement methods and apparatus of the present invention.